The “mustard oil bomb” unleashed in the seeds of sweet mignonette
“has more of a punch than Grey Poupon,” according to co-author
Denise Dearing, who is a professor of biology at the University
of Utah. It’s a good thing that the spitting mechanism kicks in,
because consumers -- including humans -- could experience health
problems if they ate too many of the seeds.

“Toxicity is all about dose,” Dearing told Discovery News. “Even
water becomes toxic at the right dose. The glucosinolates in
plants (present in sweet mignonette) in high doses can cause
goiters in humans.”

For the study, Dearing and her colleagues analyzed the
fleshy-fruited shrub, Ochradenus baccatus, which grows 3 to
6-feet tall and lives in dry streambeds found in Ethiopia, Sudan,
Egypt, Libya, Saudi Arabia and Israel east to Pakistan. The plant
produces small berries that, when mature, turn black. Each berry
holds up to 20 “mustard oil bomb” seeds. The toxic glucosinolate
substance is only released when the seeds are chewed.

Videos of spiny mice eating the fruit showed that the rodents
would deliberately wiggle the seed out of the pulp “like a person
does when eating watermelon,” Dearing said. The mice would then
spit out the seed. The spitting, however, appears to be at least
somewhat controlled by the plant.

“We don’t know what the chemical cue is that causes the mouse to
spit out the seed,” Dearing said. “(The mice) somehow know that
crushing the seed activates the toxins.”

The plant ultimately benefits, since the spitting consumer helps
to distribute the seeds in ideal locations. The mice tended to
hang out, nibble and spit in cool spots that were not in direct
sunlight. These areas are more suitable habitats for germination.
Seeds spit out by mice germinate twice as fast as seeds left
inside intact fruit.

In another experiment, the scientists “disarmed” the mustard oil
bomb by deactivating the toxic enzyme. When the fruits were then
presented to captive mice, the rodents left less than 20 percent
of the seeds intact, compared with 73 percent of the seeds that
still contained the active mustard bomb ingredient.

Yet another experiment determined that mice lost weight when they
were fed ingredients necessary for an active mustard bomb, but
not when they were fed them separately.

Camels, ibex, other rodents, lizards and many birds also
regularly eat sweet mignonette berries. All may spit out the
seeds, save for birds, which mysteriously “don’t feel the heat at
all,” Dearing said. It could just be that birds tend to not crush
seeds when feeding, allowing them to expel even the spiciest and
most toxic of seeds whole in their waste.

The findings could help to explain why so many seeds are not
tasty. Some seeds even practically fall out of the mouth. The
aforementioned watermelon seeds, for example, are not only lousy
tasting, but they also have a slippery coating that makes them
easy to spit out. Dearing said that plants with nutritious,
edible seeds, such as sunflowers, have a different growth
strategy. They “make a lot of seeds to overcome seed predation.
Some oak trees have this strategy.”

Doug Levey, program officer of Population and Community Ecology
at the National Science Foundation, told Discovery News that the
relationship between sweet mignonette and mice, in particular, is
“a beautiful example of how a fruiting plant manipulates the
behavior of rodents, turning them from seed predators to seed
dispersers.”

“The series of field observations and lab experiments make a
compelling story,” he added. “I especially like the diet study
that demonstrated when the bomb was defused, the rodents
completely switched their behavior.”